Update README.md

Remark about the AssetFactory's rootDir
Explain what workers are

I found it confusing as workers are mentioned in the "Cache Busting" section but it is not mentioned before what they are.
A bit more information about what the target path is and what CacheBustingWorker does
Reorder sections

I was at first confused what "Cache busting" and "Caching" have in common, but if I am not mistaken, they are for different purposes.

I hope this ordering makes that more clear and helps to understand what cache busting is good for.
CacheBustingWorker no longer provides two strategies and has changed the constructor's signature.

See https://github.com/kriswallsmith/assetic/pull/362#commitcomment-3208549
removed unnecessary executable bits from files
Fixed protocol-relative css imports

This fixes a mistake in the source of the CssImportFilter with protocol-relative urls. The host part of the url was lost in the process and so the import did not succeed.
Added nib on stylus filter.
Added nib on package.json.
Fixed a typo.
fixed asset collection cloning
Merge pull request #436 from mtorromeo/patch-1

Fixed protocol-relative css imports
Merge pull request #428 from mpdude/patch-1

Update README.md
Merge branch '1.1.x'
Merge remote-tracking branch 'ecentinela/master'
cleaned up nib integration